This invention relates to a sheet-like display medium utilizing rotating cylinders which makes use of an electrophoresis phenomenon, and more particularly concerns a sheet-like display medium which uses cylinders with multiple chromatic values to enable, grey scale, highlight color or a full-color display and has paperlike qualities such as being lightweight, thin, portable, flexible, foldable, high-contrast, low-cost, relatively permanent, and readily configured into a myriad of shapes as well as being able to maintain its displayed image without using any electricity.
Although paper has many desirable characteristics, unfortunately, it is not well suited for real-time display purposes. Real-time imagery from computer, video, or other sources cannot be displayed directly with paper, but must be displayed by other means, such as by a cathode-ray tube (CRT) display or a liquid-crystal display (LCD). Typically, real-time display media lack many of the desirable qualities of paper, such as physical flexibility and stable retention of the displayed image in the absence of an electric power source.
Attempts have been made to combine the desirable qualities of paper with those of real-time display media in order to create something that offers the best of both worlds and these attempts have resulted in electric paper.
Like ordinary paper, electric paper can be written and erased, can be read in ambient light, and can retain imposed information in the absence of an electric field or other external retaining force. Also like ordinary paper, electric paper preferably can be made in the form of a lightweight, flexible, durable sheet that can be folded or rolled into tubular form about any axis and conveniently placed into a shirt or coat pocket, and then later retrieved, re-straightened, and read substantially without loss of information. Yet unlike ordinary paper, electric paper can be used to display full-motion and other real-time imagery as well as still images and text. Thus it is adaptable for use in a computer system display screen or a television.
The gyricon, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper. Briefly, a gyricon is an addressable display made up of a multiplicity of optically anisotropic balls, each of which can be selectively rotated to present a desired face to an observer. For example, a gyricon can incorporate balls, each having two distinct hemispheres, one black and the other white, with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the balls are electrically as well as optically anisotropic. The black-and-white balls are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities and is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accomodate the balls, one ball per cavity, so as to prevent the balls from migrating within the sheet. A ball can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either the black or the white hemisphere to an observer viewing the surface of the sheet. Thus, by application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the balls can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image.
The gyricon is described further in the patents incorporated by reference hereinabove. In particular, U.S. Pat. No. 5,389,945 (Sheridon, "Writing System Including Paper-Like Digitally Addressed Media and Addressing Device Therefor") shows that gyricon displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, not found in CRTs, LCDs, or other conventional display media. Gyricon displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays.
Further advances in black and white gyricons have been described in U.S. patent application Ser. No. 08/716,672 titled Twisting Cylinder Display. A gyricon is described which uses substantially cylindrical bichromal particles rotatably disposed in a substrate. The twisting cylinder display has certain advantages over the rotating ball gyricon because the elements can achieve a much higher packing density. The higher packing density leads to improvements in the brightness of the twisting cylinder display as compared to the rotating ball gyricon.
Gyricons incorporating color have been described in U.S. Pat. No. 5,760,761 titled "HIGHLIGHT COLOR TWISTING BALL DISPLAY", and assigned to the same assignee, U.S. Pat. No. 5,751,268 titled "PSEUDO-FOUR COLOR TWISTING BALL DISPLAY", and assigned to the same assignee U.S. patent application Ser. No. 08/572,820 titled "ADDITIVE COLOR TRANSMISSIVE TWISTING BALL DISPLAY", and assigned to the same assignee U.S. patent application Ser. No. 08/572,780 titled "SUBTRACTIVE COLOR TWISTING BALL DISPLAY", and assigned to the same assignee and U.S. Pat. No. 5,737,115 titled "ADDITIVE COLOR TRISTATE LIGHT VALVE TWISTING BALL DISPLAY" and assigned to the same assignee.
These cases all use a spheroidal ball composed of segments arrayed substantially parallel to one another. Each segment of the ball is adjacent to at least one other segment and to no more than two other segments, adjacent segments being adjoined to one another at substantially planar interfaces. Each segment has a thickness and an optical modulation characteristic which may be different from the thicknesses and optical modulation characteristics of the other segments. The ball has an anisotropy for providing an electrical dipole moment, the electrical dipole moment rendering the ball electrically responsive such that when the ball is rotatably disposed in an electric field while the electrical dipole moment of the ball is provided, the ball tends to rotate to an orientation in which the electrical dipole moment aligns with the field.
These balls are used in a gyricon sheet. The gyricon sheet includes a substrate having a surface and spheroidal balls disposed in the substrate. Specific rotations of the balls can be made using the electrical dipole moment and can be used to make a specific segment of a ball and its associated optical modulation characteristic visible. In short, the balls are divided into segments which have different chromatic values and the balls can be rotated such that a particular chromatic value is visible. Considering the wide ranges of chromatic values, balls, and therefore gyricons, with black, white, and highlight color segments can be made, as well as balls with red, blue and green segments or cyan, magenta, and yellow segments for full color systems as well as other optical modulation properties for specialized systems.
The fabrication of the multisegmented ball employing different colors in the different fragments to be used in the gyricon sheet was a large improvement in gyricon technology because it allowed the gyricon to move from the realm of a black and white paperlike display into the realm of highlight color and full color.
However, these color gyricons suffer from several deficiencies. The first of these is a relatively low brightness, a problem inherent with rotating ball displays. The second is poor color saturation due to the way the balls are segmented and used to provide a full color display. When a base color is desired, for instance red in a red-blue-green (RBG) system, then only a small portion of the balls will be used. Using only a small portion of the balls leads to poor color saturation. The third deficiency is poor separation of the colors on the balls. When one colored segment of a ball is displayed often the adjacent colors will be visible at the edges of the segment, particularly when viewing from an off-axis position.
A color gyricon built using cylinders rather than balls has the potential to improve or eliminate these deficiencies. Closer packing arrangements are possible leading to improved brightness and contrast. Additionally, the cylindrical elements can be partitioned to reduce viewing of adjacent colors in an off-axis viewing position. Most importantly, the colors can be partitioned so that all elements will be used when viewing the optical properties leading to improved brightness and saturation for those optical properties.
A display system using a multicolor rotating cylindrical element has been patented in U.S. Pat. No. 5,526,016 and U.S. Pat. No. 5,515,075 both titled "MULTICOLOR DISPLAY APPARATUS". These patents describe a large multicolor display which has been divided into unit blocks where each block has a plurality of sections. Each section contains a multicolor display element with a rotary color display member. The color of the unit block is determined by the colors of the display elements within the unit block. The display can show a variety of colors by using various combinations of colors within the unit block. Each of the rotary color display members is affixed to shaft that also has a permenent circular magnet fixed to it. The circular magnet rotates with the rotary color display member. The rotary color display members are rotated by using magnetizing coils to selectively polarize cores in a series of stators. An electronic circuit on a circuit board is used to select and polarize individual cores, and cause individual elements to rotate.
However this system is large and bulky and requires mechanical connections and movements which require packaging in waterproof and dustproof casings to insure that the display will operate properly, especially when the display is used in outdoor conditions. If such a system could be built which eliminates mechanically rotating parts the display could be made thinner and lighter and operate more reliably when used in adverse environments.
Accordingly, it is the primary aim of the present invention to provide a display medium which has the desirable qualities of paper, qualities such as being lightweight, thin, portable, flexible, foldable, high-contrast, low-cost, relatively permanent, and readily configured into a myriad of shapes as well as being able to maintain its displayed image without using any electricity while also being able to display real-time imagery from computer, video, or other sources. Furthermore the display medium should also have good brightness characteristics and be capable of displaying multiple optical properties such as grey scale, highlight color or full color, and including (but not limited to) polarization, birefringence, phase retardation, light scattering, and light reflection with good saturation in the individual optical properties. Additionally the display medium should be as self-contained as possible with a minimum of mechanical connections or interfaces.
Further advantages of the invention will become apparent as the following description proceeds.